Nematicity in iron pnictides: phase competition and emergent symmetry

TL;DR

This paper investigates the interplay of nematic and charge orders in iron pnictides, revealing an emergent U(1) symmetry at a first-order transition that leads to a Goldstone mode and soft excitations, with experimental support.

Contribution

It uncovers an emergent U(1) symmetry at the nematic-charge order transition and characterizes the hidden algebra linking these orders in iron pnictides.

Findings

Emergent U(1) symmetry at the phase transition.

Presence of a Goldstone mode near the transition.

Softening of excitations in nematic and charge sectors.

Abstract

The phase diagram of iron-based superconductors contains a host of electronic orders, which are intimately connected with their superconductivity. Here we analyze the fluctuations of one type of nematic order in another. Our analysis leads to an emergent U(1) symmetry at a first-order transition between a nematic phase and a $C_4$-symmetric charge-ordered phase. We characterize the continuous symmetry in terms of a certain hidden Lie algebra that links the different orders. This emergent symmetry leads to a Goldstone mode at the transition and causes softening of excitations in the nematic and charge sectors near the transition. The underlying physics bears a resemblance to the anisotropic XZ spin model, with the nematic order and charge $C_4$ order parameters playing the roles of the $x$ and $z$ components of the magnetization vector, respectively. We provide the experimental evidence in support of the proposed effects, and discuss the general implications of our results for the physics of iron-based superconductors and other correlated systems.